Relationship between the Photocatalytic Characteristics and the Oxygen Partial Pressure of TiO₂ Thin Films Prepared by a DC Reactive Sputtering Method

Abstract

It was confirmed that the oxygen partial pressure that is used during DC reactive sputtering can influence the generation of oxygen defects on TiO 2 thin films and/or the surface structure, which then affects the photocatalytic characteristics. TiO 2 films were deposited by DC reactive magnetron sputtering on a Si wafer at 250°C under oxygen partial pressures of 0.1 Pa, 0.5 Pa and 0.8 Pa. Structural analysis of the TiO 2 thin films was conducted using X-ray diffraction (XRD), field-effect scanning electron microscopy (FE-SEM), X-ray photoelectron spectroscopy (XPS), the photo luminescence method (PL). Characterization of the photocatalytic properties was performed by measuring the concentration change of methylene blue (wet methylene blue method) and the change of the contact angle with pure water. All films showed a polycrystalline anatase structure. TiO 2 thin films deposited under reducing conditions (an oxygen partial pressure of 0.1 Pa) exhibited peaks that are characteristic of Ti 3+ and can also contain a number of oxygen defects. The domain size was about 150–300 nm, and the domain boundaries were unclear. On the other hand, for TiO 2 thin films deposited in oxidizing mode (oxygen partial pressures of 0.5 Pa and 0.8 Pa), the peak corresponding to Ti 3+ was not observed. The domain size was between 300–1000 nm and the boundaries were clear. TiO 2 thin films exhibit better photocatalytic properties when the domain size is large and there are fewer oxygen defects in the film. It has been suggested that the decreasing proportion of domain boundaries, which act as trapping centers for electrons, and the lower concentration of oxygen defects, which act as recombination centers for electron-hole pairs, improved the photocatalytic characteristics.